The present invention relates generally to an apparatus for producing fluorescence in the gases from a reactor chamber.
Optical emission spectroscopy (OES) endpoint detection relies on the change in intensity of light emitted from the gas species involved with the etch process. During steady-state etching of the etch layer, the reactant gases and reaction gas products are excited by the plasma causing fluorescence of light at wavelengths characteristic of those species. When the etch layer is exhausted, the consumption rate of reactant gases and production rate of effluent gases change. Those changes cause the intensity of light from one or more emission wavelengths to change. Monitoring those wavelengths and appropriately combining the signal changes produce the familiar OES endpoint trend plot that is used to signal the end of the process step. Recent new plasma etch processes have been encountered in which this traditional monitoring method is failing to detect the endpoint. There seems to be at least two reasons for this failure.
“Remote plasma” processes use a plasma upstream of the location at which the etch process is to occur. The chemically activated species flow into the process chamber and successfully remove deposition films from every surface in the deposition chamber. However, neither the reactive species nor the effluents of the etch process are excited into high enough energy state(s) needed to produce fluorescence. The absence of fluorescence in the reaction chamber is the reason these “dark plasma” processes fail to produce a traditional OES endpoint.
Low energy plasmas are being used in new processes. Although these processes do excite the gas species to fluorescence that can be detected in the process chamber, the resulting spectra do not display the one or more wavelength intensity changes required for successful OES endpoint detection. This kind of failure appears to be different from the usual problem of low percent open etch area. The exact reasons that the endpoint signal is not detectable are not well understood.
A traditional OES endpoint signal for both of these problem processes has been demonstrated by striking a secondary plasma in the effluent gases downstream of the reaction chamber of the wafer plane. Success has been reported when attaching the secondary plasma unit to the side of the etch process chamber just before the throttle valve. Others have reported success placing the secondary plasma downstream from the turbo pump—a less well regulated pressure environment that can cause OES signal fluctuations. These “secondary plasma” units are commercially available. However, they suffer from two problems.
The OES signal is collected through a window that is relatively close to the secondary plasma. The energy of the plasma is such that it will break the effluent gases into small molecular components that will recombine on surfaces to form a polymer—a common problem in carbon halide etch chemistries. The polymer on the OES observation window attenuates the light signal so rapidly as to make the method impractical for a manufacturing tool requiring long mean times between maintenance.
Practitioners report that it is difficult to control the secondary plasma such its excitation of the effluent gas is stable over long periods of time. This is not surprising considering the heroic measures the OEM makes to produce a stable and repeatable plasma for processing. The cost constraints for a secondary plasma unit prohibit the use of all those same sophisticated techniques for producing stable reproducible plasma performance. Changes in the excitation of the effluent gas caused by fluctuations in the secondary plasma changes the light intensity that can be misinterpreted as changes in the manufacturing process being monitored.